Chapter 6, Section 3
Key Concept: Substances’ properties depend on their bonds.
BEFORE, you learned
• Chemical bonds hold the atoms
of compounds together
• Chemical bonds involve the
transfer or sharing of electrons
• Molecules have a structure
NOW, you will learn
• How metal atoms form chemical
bonds with one another
• How ionic and covalent
bonds influence substances’
properties
Metals have unique bonds.
Metal atoms bond together by sharing their electrons with one another.
The atoms share the electrons equally in all directions. The equal
sharing allows the electrons to move easily among the atoms of the
metal. This special type of bond is called a metallic bond.
The properties of metals are determined by metallic bonds. One
common property of metals is that they are good conductors of
electric current. The electrons in a metal flow through the material,
carrying the electric current. The free movement of electrons among
metal atoms also means that metals are good conductors of heat.
Metals also typically have high melting points. Except for mercury, all
metals are solids at room temperature.
Copper and other metals get their properties from metallic bonds.
The ability of electrons to move freely makes metals
• good conductors of electricity
• good conductors of heat
• easy to shape
Metallic Properties
Two other properties of metals are that they are easily shaped by
pounding and can be drawn into a wire. These properties are also
explained by the nature of the metallic bond. In metallic compounds,
atoms can slide past one another. It is as if the atoms are swimming in
a pool of surrounding electrons. Pounding the metal simply moves
these atoms into other positions. This property makes metals ideal for
making coins.
reading What three properties do metals have because of
metallic bonds?
check your
Ionic and covalent bonds give compounds certain
properties.
The properties of a compound depend on the chemical bonds that
hold its atoms together. For example, you can be pretty certain an
ionic compound will be a solid at room temperature. Ionic compounds,
in fact, usually have extremely high melting and boiling
points because it takes a lot of energy to break all the bonds between
all the ions in the crystal. The rigid crystal network also makes ionic
compounds hard, brittle, and poor conductors of electricity. No
moving electrical charges means no current will flow.
Ionic compounds, however, often dissolve easily in water, separating
into positive ions and negative ions. The separated ions can move freely,
so solutions of ionic compounds are good conductors of electricity.
Your body, in fact, uses ionic solutions to help transmit impulses
between nerve and muscle cells. Exercise can rapidly deplete the body
of these ionic solutions, and so a good sports drink contains ionic
compounds like potassium chloride that replace the ions lost during
physical activity.
Mineral hot springs, like those found in Yellowstone National
Park, are another example of ionic solutions. Many of the ionic compounds
dissolved in these hot springs contain the element sulfur,
which can have an unpleasant odor. Evidence of these ionic compounds
can be seen in the white deposits around the pool’s rim.
Covalent compounds have almost the exact opposite properties
of ionic compounds. Since the atoms are organized as individual
molecules, melting or boiling a covalent compound does not require
breaking chemical bonds. Therefore, covalent compounds often melt
and boil at lower temperatures than ionic compounds. Unlike ionic
compounds, molecules stay together when dissolved in water, which
means covalent compounds are poor conductors of electricity.
Table sugar, for example, does not conduct an electric current when
in solution.
Bonds can make the same element look different.
Covalent bonds do not always form small individual molecules. This
explains how the element carbon can exist in three very different
forms—diamond, graphite, and fullerene. The properties of each form
depend on how the carbon atoms are bonded to each other.
Diamond is the hardest natural substance. This property makes
diamond useful for cutting other substances. Diamonds are made
entirely of carbon. Each carbon atom forms covalent bonds with four
other carbon atoms. The pattern of linked atoms extends throughout
the entire volume of a diamond crystal. This three-dimensional
structure of carbon atoms gives diamonds their strength—diamond
bonds do not break easily.
RESOURCE CENTER
Another form of carbon is graphite. Graphite is the dark, slippery
component of pencil “lead.” Graphite has a different structure from
diamond, although both are networks of interconnected atoms. Each
carbon atom in graphite forms covalent bonds with three other atoms
to form two-dimensional layers. These layers stack on top of one
another like sheets of paper. The layers can slide past one another easily.
Graphite feels slippery and is used as a lubricant to reduce friction
between metal parts of machines.
A third form of carbon, fullerene, contains large molecules.
One type of fullerene, called buckminsterfullerene, has
molecules shaped like a soccer ball. In 1985 chemists made a
fullerene molecule consisting of 60 carbon atoms. Since then,
many similar molecules have been made, ranging from 20 to
more than 100 atoms per molecule.
Questions for Chapter 6, Section 3
KEY CONCEPTS
1. How do metal atoms bond together?
2. Why do ionic compounds have high melting points?
3. What are three forms of the element carbon?
CRITICAL THINKING
4. Apply A compound known as cubic boron nitride has a structure
similar to that of a diamond. What properties would you
expect it to have?
5. Infer Sterling silver is a combination of silver and copper.
How are the silver and copper atoms held together?
CHALLENGE
6. Infer Why might the water in mineral springs be a better
conductor of electricity than drinking water?